Field of the Invention
The present invention relates to a solid-state imaging device having an AD conversion unit that converts a pixel signal to a digital signal.
Description of the Related Art
As an analog-digital converter (ADC) that makes an analog-digital (AD) conversion of a signal output from each pixel of a CMOS image sensor, there is proposed a ramp-compare column ADC in which an AD conversion function is provided for each column of a pixel array. The ramp-compare column ADC has a comparator provided for each column, and a reference signal generation unit. The comparator compares the signal level of a pixel signal with the signal level of a ramp signal as a reference signal output from the reference signal generation unit. Then, a count value corresponding to a time that elapses before the magnitude relationship between the signal level of the pixel signal and the signal level of the ramp signal is reversed is held as digital data in a column memory provided for each column.
For example, Japanese Patent Application Laid-Open No. 2013-085103 teaches that plural count signals indicating a count value are output from a counter, and the count signals at the timing of inverting the magnitude relationship between the signal level of the pixel signal and the signal level of the ramp signal is held in a storage unit for each column.
Recently, the demands to increase the speed of a count signal from a standpoint of speeding up readout operation, and to increase the scale of a column circuit from a standpoint of enhancing the definition have been increased. Therefore, a delay difference between bits and a variation in duty ratio of a count signal have largely affected the image quality, and hence the quality of the count signal has been required to be managed more appropriately.